Over the years, a wide variety of network technologies have been developed to carry various types of information. Early networks were designed with voice communications in mind. These networks were, and still are, primarily circuit-based networks. Time Division Multiplexing (TDM) is a common circuit-based network technology. In recent decades, the wide spread use of computers has led to the development of networks designed with data communications in mind. These data networks are primarily packet-based or cell-based. Examples of packet-based or cell-based network services include frame relay, ATM (asynchronous transfer mode), and IP (internet protocol).
Due to the wide spread usage of so many different kinds of network technologies, network providers have had to maintain several different networks to satisfy the needs of network users. For instance, a corporation may have invested millions of dollars in a private frame relay network. If the corporation wants to connect offices in different cities to the private network, the corporation can contract with a telephone company to provide frame relay services between the offices over a public frame relay network. Another corporation may have invested millions of dollars in a private ATM network. If the telephone company maintains public networks for both frame relay and ATM services, the second corporation can also contract with the telephone company to connect offices in different cities. Of course, the telephone company will probably also have to maintain TDM services for voice communications as well as IP services for public internet traffic.
Maintaining all of these public networks is very expensive. Network providers would prefer to maintain just one public network. Many network users, however, have invested heavily in particular types of private network services and are therefore reluctant to modify or abandon existing networks to accommodate the desires of network providers.